When forming products by deep drawing sheet metal, gas springs are used both to hold the metal sheet and to assist in separating tool halves from each other after a forming operation. Gas springs which support the sheet metal holder are loaded in the beginning of the press cycle and relieved afterwards. The extent to which each gas spring is loaded is determined by the press cycle, the form of the pressed product and of the effective stroke length. For the sheet metal forming operation to be optimal, it is necessary to have complete control of the press cycle.
There is a risk that the press cycle cannot be controlled in a satisfactory manner. Gas springs may be adversely affected in connection with an overstroke, i.e. a compression stroke that went too far, exceeding the nominal length of stroke of the gas spring. Such overstroke may cause damage to the parts making up the gas spring, including the cylinder, the piston rod, the piston and the guide, thus leading to reduced length of life, or at worst, rupture. Furthermore, and perhaps more seriously, press tools and other types of equipment may become damaged if gas springs are overstruck.
To avoid this type of problems, there are gas springs having safety devices, such as the ones disclosed in EP0959263B1 and EP1241373B1, where a safety plug is mounted in the bottom part of the cylinder.
However, with such safety plugs, situations may arise where the safety device is accidentally and unintentionally triggered. One such situation is where the gas springs are intentionally relieved of gas, e.g. in connection with service. The weight of the sheet metal holder may accidentally compress the empty or near empty gas spring such that it reaches its end position and triggers the overstroke protection.
Hence, there is a need for a safety device which is improved in this respect.